The present invention relates to the computer display of a video signal transmitted over an asynchronous network.
Recent developments in computer technology enable users of remote computers to communicate using interactive digital video; see, for example, the March 91 issue of PC Week. Each user has a personal computer or workstation equipped with a card, such as the Intel/IBM ActionMedia II (AMII) card, to allow the computer to process and display video images. (IBM is a trademark of the IBM Corporation, and Intel and ActionMedia are trademarks of the Intel Corporation.) The workstations are connected together by a relatively wideband communication channel, such as an Integrated Services Digital Network (ISDN) or Local Area Network (LAN). A camera at one workstation provides a video image which is transmitted, in compressed form, across the network. This video signal is received and displayed at one or more other workstations to provide real-time visual communication between users.
Traditionally, video signals have been transmitted over fully synchronous or isochronous links. In such systems a camera obtains a video signal at a fixed frame rate, which is then transmitted to and displayed at a receiving station, all at the same fixed rate. By contrast, computer networks provide asynchronous communication in which information is usually transmitted in individual packets to allow any loss or corruption of data to be detected. The time between dispatch and receipt of a packet varies according to the amount of traffic on the network and other factors.
In computer-based video communication systems, a video signal is obtained from the camera at a constant frame rate but, after transmission across the asynchronous or non-ideal network, the frames arrive at irregular intervals. Some frames arrive early, some are delayed, and bunching can occur. The display device at the receiving terminal, however, generally requires a constant frame rate supplied to it (e.g., to match the raster scan rate of a CRT). In such systems it is therefore necessary to match the irregular arrival of frames over the network with the constant supply required to the output screen.
It is known in multimedia systems, in which video sequences are read from optical disks, to compensate for the mismatch in rate between data coming from the disk and the display of the images by filling a buffer with frames prior to play-out to the display device. However, it is difficult to adopt this approach in video conferencing applications, since each frame stored in the buffer adds to the delay between capture and final display of a video image. Too large a delay is very intrusive in interactive communications. The designer of computer based video communication systems is therefore faced with the problem of how to achieve regular play-out of the asynchronous incoming video signal while, at the same time, minimising the number of buffered video frames.